i965: Update physical width/height munging for 2x IMS MSAA.

I can't find any documentation to explain what ought to be done here, so
I simply guessed based on the pattern I observed in the 4x/8x cases.
It appears to work, but it could be totally wrong.

I was able to find the Sandybridge PRM quote from the comments in the
latest documentation: Shared Functions > 3D Sampler > Multisampled
Surface Behavior.  However, it only mentions 4x MSAA - not even 8x.

After a substantial amount more digging, I was able to find a second
page (incorrectly tagged) which confirmed the formulas in our code for
8x MSAA.  However, that page didn't mention 2x MSAA at all.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Anuj Phogat <anuj.phogat@gmail.com>

src/mesa/drivers/dri/i965/intel_mipmap_tree.c

@@ -313,6 +313,11 @@ intel_miptree_create_layout(struct brw_context *brw,
           * sample 3 is in that bottom right 2x2 block.
           */
          switch (num_samples) {
+         case 2:
+            assert(brw->gen >= 8);
+            width0 = ALIGN(width0, 2) * 2;
+            height0 = ALIGN(height0, 2);
+            break;
          case 4:
             width0 = ALIGN(width0, 2) * 2;
             height0 = ALIGN(height0, 2) * 2;
@@ -322,7 +327,7 @@ intel_miptree_create_layout(struct brw_context *brw,
             height0 = ALIGN(height0, 2) * 2;
             break;
          default:
-            /* num_samples should already have been quantized to 0, 1, 4, or
+            /* num_samples should already have been quantized to 0, 1, 2, 4, or
              * 8.
              */
             assert(false);